The invention relates to lifting surfaces, such as wings, operating at subsonic speeds and more specifically, to lifting surfaces influenced by the flow of air from vortices located in close proximity to the trailing edges of the lifting surfaces.
An idealized airfoil, such as a flat plate of infinite span and a thickness approaching zero, when moving through a gaseous fluid, e.g., air, at a fixed velocity, and the surface of the airfoil is at a small angle, i.e., the angle-of-attack, relative to the direction of motion, the oncoming flow is separated into a flow of air along the upper surface of the airfoil and a flow of air along the lower surface. This bifurcation of the flow starts in the vicinity of the leading edge of the airfoil, and becomes confluent at the trailing edge of the airfoil. This Kutta Joukowsky hypothesis is acceptably accurate for the ideal airfoil, provided the angle-of-attack of the airfoil approaches zero. For much larger angles-of-attack, the velocity of the airflow over the upper surface of the airfoil is considerably reduced as the flow of air approaches the trailing edge. The air, therefore, separates from the wing simply because the reduced momentum of the air flow prohibits the flow of air to continue to the trailing edge of the wing and beyond. This flow separation results in loss of wing lift and can cause an aircraft flight safety problems. Severe causes of wing air flow separation is commonly termed xe2x80x9cwing stallingxe2x80x9d. Practical airfoils, or wings, do not have infinite spans. Airfoils of finite wingspan, therefore, when moving through the air at a finite speed and inclined to the direction of motion, i.e., having a positive angle-of-attack, will be pushing down on the incoming air. The reaction of the affected air is to impose an air pressure on the lower surface of the airfoil and in close proximity to the wingtips to accelerate air upward and around the wingtip edges of the finite span wing. The air moved upwards in the vicinity of the wing tips, as the wing moves forward, forms a rolled-up vortex flow whose axis of formation is nearly parallel to the direction of flight. Because wingtip vortices are known to increase the drag of an airfoil and with it, a reduction in its aerodynamic efficiency, the reduction of wingtip vortices is a subject of continued practical interest. Airfoils moving in a rotational manner are also known to produce vortex flows off the edges of their distal wingtip, or blade tip. Due to the reduction in efficiency produced by blade tip vortices, airfoil shapes, particularly their planforms, are varied in efforts to minimize vortex production. Two or more airfoils rotating about a hub or axis of rotation can be termed a rotor or a propeller with the word propeller applied principally to the rotating propulsor providing forward thrust.
Lifting structures can include the combination of a translational airfoil, a wing, and one or more rotors in close proximity to the trailing edge of the wing. Rotors in close proximity to a wing provide lift augmentation to the wing, drag reduction and stall resistance. If mounted and articulated, they may provide flap augmentation.
Rotors surrounded or partially surrounded by the surfaces of a wing are known to interact via the vortex flow of their blades with the airflow of the wing. An example of this aerodynamic interaction is the augmentation of the lift of the wing of an aircraft by the interactions of the wing flow with flow of a rotor located in a semi-circular cutout in the rear portion of the wing with the center of rotation of each rotor located on what would otherwise be the trailing edge of the wing.
While the above art provides for enhanced lift and reduction in drag to airfoils, there remains a need for significant basic improvements in the aerodynamic performance of wing-rotor systems, including wing stalling. New technical design architectures for vehicular embodiments and operation are required, including the synergistic combination of a wing, rotor and propeller for increased lift, reduced drag and increased thrust requirements. In particular, when compared to semi-circular cutouts, there is a significant fundamental need to increase the wing area ahead of the cutout so that a larger portion of the wing area from the wing""s leading edge to cutout contour, is more favorably influenced by rotor flow. Accordingly, there is a need to move the axis of rotor rotation aft of the wing""s original trailing edge thereby increasing the surface area of the wing ahead of the frontal section of the cutout contour. Further and most significantly, moving the axis of rotation aft of the wing""s trailing edge also minimizes and possibly eliminates any potential adverse effects of rotor inflow on induced wing lift and stall resistance from that portion of the rotor inflow that is located behind the axis of rotation of the rotor.
Each blade of a propeller produces a vortex flow that generally is not considered by those skilled in the art as a contributor to induced lift or induced thrust. There are geometric techniques, as stated herein, for propellers located in close proximity to wing surfaces that permit propeller blade tip vortices to augment wing lift and provide induced thrust.
Disclosed are the methods and means for enhancing lift, reducing drag, and enhancing the pre-stall angle-of-attack of an airfoil illustrated by several apparatus embodiments of the present invention wherein wing-rotor configurations are described that provide aerodynamically enhanced high lift, low drag, and a resistance to stall. Also described is a wing leading edge air blowing system in conjunction with a trailing edge wing-rotor combination providing means for substantially vertical take-off and landing of air vehicles and substantially precluding the air vehicle from stalling. In addition, wing-rotor-propeller embodiments are described that provide aerodynamically high lift and low drag and forward thrust for all vehicles including embodiments for air vehicles and watercraft.
The teachings of the present invention illustrate, through example embodiments, the benefits of vortex interaction maximization via less-than-semicircular airfoil cut-outs (i.e., arcular or arcuate airfoil trailing edge recesses) while enhancing the vortex amplitude via rotor blade planform design. Additionally, the present invention through example embodiments, illustrates the benefits of vortex interaction maximization using a propeller interposed between two substantially parallel airfoils with at least one airfoil having a plurality of rotors each rotating within a cut-out or recess. Additionally, the present invention, through example embodiments, illustrates the benefits of augmenting wing-rotor assemblies with air blowing devices and thereby enhance the lift augmenting of the wing-rotor assembly.